The present invention relates generally to the transformation and use of algal cells for the production of metabolic products and, in particular, to the use of transformed algal cells for producing ethanol.
Current approaches to use and management of energy resources have several associated problems, such as pollution and CO.sub.2 generation, but cost remains a key consideration. Oil and coal continue to be less expensive, at least in the short term, than alternative sources. Attempts have been made to utilize other energy sources, but competitive methods remain marginal. An attractive approach is to use solar power, since the input energy is free and inexhaustible. The cost of energy harvesting solar power, however, is prohibitively high with present technology so that it is used primarily in remote areas where less expensive energy sources are unavailable.
Plants utilize light energy, and they are by far the least expensive means of converting light energy to chemical form. The problem with using plants as an energy source derives from the costs and inefficiencies associated with harvesting, processing and conversion to usable form. If these problems can be overcome, then energy farming will be economical.
Techniques for obtaining energy from plant sources include fermentation of crops to ethanol (Winston (1981), Ethanol fuels, Solar Energy Research Institute) including corn, other grains, potatoes, sugar cane and sugar beets, and fermentation to methane (Wise (1981) Fuel Gas Production from Biomass, CRC Press, Boca Raton) or ethanol (Lynd et al. (1991) Science 251:1318-1323) of biomass, including wood, crop residues, and algae.
One of the more successful of these approaches is sugar cane fermentation. In the U.S., corn is an economical starting material (Winston (1981) supra). Biomass fermentation is economical when starting materials are waste with a very low or negative value, but it is not a competitive use of farmland. Biomass fermentation to ethanol is expected to become more economical as a result of technical advances, and it may contribute substantially to our energy budget in the future (Lynd et al. (1991) supra).
Algal systems for use in fermentation suffer from two disadvantages: the low value of the final product and high production costs. Microalgae require constant stirring to prevent adhesion to pond bottoms and must be harvested by filtration of very large volumes of water. Macroalgae methods have excessive capital costs, principally for harvesting equipment, Bird, K. T., Benson, P. H., eds., 1987. Seaweed cultivation for renewable resources, Elsevier, Amsterdam.